Amelia grins with a slight hint of crazy behind her eyes, stepping onto her flying carpet to rise into the air, leaving one of her arcane cannons on the ground for support while bringing the ither with her.

Initiative: 13

Kass sees all of the villains arrayed before him, and he'll say "Time for a little divine intervention, wouldn't you say?"  

During the prep round, he'll cast Summon Celestial and choose the Defender form.  

Initiative: 29

“Won’t live to see the day? Fine with that, already half dead anyways. Now, this is gonna be one hell of a show!” Evelyn says, casting mirror image on herself and drawing her sword in one hand.

Seems like the only people who haven't roll initiative are Evelyn and Ruin, so I'll roll for them real quick:

Evelyn: 13

Ruin: 18

Initiative Order:

Divine Master > Amari > Kass > Ruin > Arcane Master > Wripers > Evelyn > Primal Master > Amelia

Divine Master's Turn!

The Master summons a spiked hammer made of shadow into their hand, a glowing eldritch weapon of destruction. With a shout, they raise the hammer, causing twisting gears to burst out from the weapon. The gears affect the two nearest party members, which are...

Which are....Evelyn and Ruin. You both must make a DC 25 DEX save or: 

The Divine Master dashes forward, flying on their scorched wings. With silent precision, they swing their hammer twice.

Hammer Attack #1 VS. Wripers: 27 to hit, 50 bludgeoning plus 4 force damage

Hammer Attack #2 VS. Amari: 19 to hit, 31 bludgeoning plus 11 force damage

Amari, GO! (the Divine Master is next to you, the Primal and Arcane Masters are both about 30ft away.) 

Evelyn’s save: 17

Quote from TheFriendlyArchfey >>

Evelyn’s save: 17

 Keep in mind, Amari cast bless on the whole party, which means you get to add an extra D4. Won't help much in this circumstance, since you rolled a 17, but will be useful later on. REMEMBER: everyone except Amari has a bless affecting them!

I really need to stop editing my posts

Dex save: friggin' 6. I don't think that d4 from Bless would make the slightest bit of difference.

Both Ruin and Evelyn take the full damage and are pushed away.

Amari, GO!

 Amari reels momentarily from the force of the hammer strike. She then steadies herself and calls down a column of fire upon the Divine Master...

• Flame Strike (cast at 8th level): 11 fire damage + 22 radiant damage, both of which can be halved on a successful DC24 DEX save

Divine Fire roars down, searing the flesh of the Divine Master [Dex Save: 30]

And, Wripers, the casting of a offensive fire spell is signal enough to your mind.

Quote from Raccoon_Master >>

Divine Fire roars down, searing the flesh of the Divine Master [Dex Save: 30]

And, Wripers, the casting of a offensive fire spell is signal enough to your mind.

 [What mind?]

Quote from BoldHand22 >>

Quote from Raccoon_Master >>

Divine Fire roars down, searing the flesh of the Divine Master [Dex Save: 30]

And, Wripers, the casting of a offensive fire spell is signal enough to your mind.

 [What mind?]

 Lol. The master succeed on their save, taking half damage (19 total)

Kass decides to join his companions and focus his attention on the Divine Master.  He'll cast Disintegrate at 7th level, a sickly green beam of light extending from his finger and streaking toward the Divine Master.  He then yells for his new Celestial friend to strike the Divine Master as well.

Disintegrate:  DC19 Dex save, or take 77 force damage.

Celestial's attacks:

Attack 1: 21 Damage: 14  and Ruin gains 4 temporary hp
Attack 2: 16 Damage: 10  and Amari gains 3 temporary hp

Dex Save: 18

The celestials first attack misses, parried by the hammer. The second attack lands true though, and Amari feels filled with celestial vigor as she gains 10 Temp HP.

Ruin, GO! (you are blessed, remember that)

Yo! Missed by one!

The Divine Master is struck by the disintegrate spell, causing part of their flesh to be well...disintegrated.

Ruin is rather annoyed (in a cold, mechanical way) that his icy armor casing just got shattered in one blow. He decides that his top priority right now shall be defense, as he casts Fizban's platinum shield on himself with his bonus action.

With his action, he'll cast lightning lure at the divine master, who must now make a DC 19 Strength save or take 16 lightning damage (rolled in game log) and be dragged 10 ft closer to the automaton.

Ok, I want to make one thing clear. This may be my fault if I didn't convey the vibe correctly, but this is the first of 2 boss fights you while have for this "adventuring day". This is the harder one, and since you are level 20, you have abundant resources. Don't be afraid to go all on these 2 boss fights. You will get a rest after the two boss fights. Or at least you while have an opportunity. Its your choice whether you want to rest. For most of this adventure, time is money (or more accurately, your life), but for now, don't be afraid to spend a buch of expensive resources.

Strength Save: 15

Lightning crackles around Ruin, twisting around his foe. A shimmering force-bubble of energy envelops the warforged, providing him protection from the coming onslaught.

Arcane Masters Turn!

The Master waves their hand, telekinetically summoning a curved, shimmering metal blade to their grasp. With a sneer and a draconic roar, they charge forward, sweeping their blade in a cleaving arc. In their other hand they conjure a ball of flame and radiance, which they first use to ingite their sword and then send hurling at Amari & Amelia.  Both of you must succeed on a DC 25 Dex Save or:

Sword attack #1 VS. Wripers: 25 to hit, 23 Slashing plus 19 Fire

Sword [action]attack[/action #2 VS. Amari: 14 to hit, 26 Slashing plus 22 Fire

Wripers, GO! After Wripers is Evelyn (both of you are blessed. The Divine and Arcane Masters are next to Wripers, and Evelyn is away from all three masters)

*slight addendum, the Fireball was only supposed to be 48 fire damage.

The lightning fails to ensare the Divine Master, who dodges out of the way. Their hammer melts, and the raw unshaped essence congeals in their hands.

Wripers, losing all preconception of a plan, let's out a vicious(ly plucky squeal of a) battle cry, inducing his rage. He directs this rage towards the Arcane Master, swinging his hammer with undiluted force...!

[First swing to hit: 31 (17{+14})]

[Damage: 33 (18 {+11 radiant <+4 rage>})]

His second strike comes on his backswing, not quite as accurate, but certainly just as furious.

[Second swing to hit: 19 (5{+14})]

[Damage: 17 (13 {+4})]

The wicked little smile on his long thin maw suggests he's having fun. It could also be gas from that manticore earlier this morning.